The concentrations of glycogen, glucose, ATP, glucose-6-phosphate and lactate were studied in normal and cryptorchid rat testes using enzymatic pyridine nucleotide methods. With the exception of ATP, all the substrates were significantly higher in the cryptorchid testes. The decrease of the energy metabolites and the accumulation of lactate were also studied in testes incubated in the absence of O2 at the temperatures of 33·6° C and 36·6° C. The metabolic rate of the incubated organs was estimated in terms of the rate of use of high energy phosphate ( ~ P) calculated from changes in ATP, glucose and glycogen. A good parallelism between hexose utilization and lactate production was observed in both normal and cryptorchid testes. A rapid mobilization of glycogen, suggesting activation of phosphorylase, was observed in incubated cryptorchid testes. The calculated oxygen consumption of cryptorchid testes was about 100% higher than that of the normal testes at the same temperature. It is suggested that the basic metabolic rate of the interstitial tissue is higher than that of the seminiferous tubules.